AN OVERVIEW OF THE IODINE BEHAVIOUR IN THE TWO FIRST PHEBUS TESTS FPT-0 AND FPT-1

D. JACQUEMAIN, C. POLETIKO, C. WREN, D.A. POWERS, S. DICKINSON, Y. DROSSINOS, F. FUNKE, B. HERRERO and R. CRIPPS "AN OVERVIEW OF THE IODINE BEHAVIOUR IN THE TWO FIRST PHEBUS TESTS FPT-0 AND FPT-1 ”" CSNI : Workshop on iodine aspects of severe accident management, Helsinki, Finlande (10-20 May 1999)

PHEBUS FP is an international programme which objective is to investigate key phenomena involved in LWR severe accidents. The first test FPT-0 was performed in December 1993 with trace irradiated fuel. The second test FPT-1 was performed in July 1996 with re-irradiated fuel. This paper presents an overview of the iodine behaviour ob-served in both tests and discusses the implications for the modelling of the iodine chem-istry in the containment of PWR. The results of FPT-1 confirm the main features highlighted in FPT-0 : - the presence of gaseous iodine in the containment early during the degradation phase due to direct injection of gaseous iodine from the primary circuit, - the rapid deposition of iodine in the sump as insoluble Agl, and consequently no signifi-cant revolatilisation of iodine from the sump due to radiolysis was observed, even though the sump pH was acid, - indication that the gaseous iodine on the long term is produced by release from con-tainment surfaces (potential mechanism : organic iodides production from painted con-densor surfaces), producing a stable gaseous iodine concentration. The data obtained from the two experiments indicates that the current modelling in iodine chemistry codes (iodine concentration and partition determined from formation of volatile iodine in the sump, transfer to containment atmosphere, loss by deposition on surfaces) is inadequate to represent the PHEBUS tests results. The comparison of mod-elling and experimental data has identified areas of necessary developments (besides the need of a better understanding of primary circuit chemistry) that include the modelling of kinetics of organic iodides formation/decomposition and of Ag/l interactions describing Ag chemistry in the containment (in particular oxidation process), Agl formation and stability. These developments, amongst others, will be based on experimental data gathered in European Community programs and PHEBUS Project supported programs, described in the paper. Partner organisations: R. Cripps Accident Progression Section Paul Scherrer Institute CH-5232 Villigen, Switzerland C. Wren Research Chemistry Branch AECL/Whiteshell Laboratories Pinawa, Manitoba, R0E1L0, CANADA D. Powers Sandia National Laboratories Nuclear Facilities Safety P.O. Box 5800, Mail Stop 0744, Albuquerque, NM 87185, USA Y. Drossinos CEC Joint Research Center, 21020, ISPRA, ITALY F. FunkeNuclear Power Generation Division, SIEMENS/KWU, D-91058 Erlangen, GERMANY B. Herrero CIEMAT Madrid Avda. Complutense 22-28040 Madrid, SPAIN